Stretch-forming device and method for stretch forming a workpiece and stretch-formed part

ABSTRACT

A stretch-forming device is provided for stretch forming a workpiece, which has a first bending device having a first bending stamp and a first bending shoe and a second bending device having a second bending stamp and a second bending shoe. In addition, the stretch-forming device has a first shaft and a second shaft, the first shaft mounted so it is rotatable around a first axis and providing a first support surface for the workpiece, and the second shaft mounted so it is rotatable around a second axis and providing a second support surface for the workpiece. The first bending stamp and the second bending stamp are components of a first part of the stretch-forming device, and the first bending shoe and the second bending shoe are components of a second part of the stretch-forming device. The first part and the second part are movable relative to one another. The second part has a first recess and a second recess, the first bending shoe and the first shaft situated on opposite sides of the first recess and the second bending shoe and the second shaft situated on opposite sides of the second recess.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to German Patent Application No.102009056315.6, filed Dec. 1, 2009, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The technical field relates to a stretch-forming device for stretchforming a workpiece, a method for stretch forming a workpiece using astretch-forming device, and a stretch-formed part of an outer skin of amotor vehicle.

BACKGROUND

A stretch-forming tool having a tool lower part and a tool upper part,which is movable relative thereto, a forming stamp, and a stretchingdevice, which can be lowered in relation to the forming stamp, clampsthe sheet-metal board to be deformed at the edges, and forms it undertensile stress until it reaches the plastic state on the forming stamp,is known from DE 102 22 314 A1. In this stretch-forming tool, akinematically simple, compact construction and precise control duringprocessing of the stretch forming is achieved in that stretching slides,which work together in pairs via terminal clamping surfaces runningtransversely to the stroke direction of the tool parts, are provided,and these are each situated so they are displaceable in the tool lowerpart or upper part on linear guides inclined at equal angles diagonallyto the tool interior and are movable in the retraction directionopposite to a defined counter force, which causes the clamping of theboard between the clamping surfaces.

A device for planar stretch forming of workpieces is known from DE 102006 047 484 A1, which has a tool upper part and a tool lower partengaged with the tool upper part, which are movable relative to oneanother. Both the tool upper part and also the tool lower part each haveat least two reforming beads, which are spaced apart from one another,on at least two sides, which each engage in corresponding recesses ofthe tool lower part or the tool upper part.

Outer skin parts that are produced by stretch forming according to theprior art have a relatively slight pre-stretching, for example, 0.5% to2% pre-stretching. A higher pre-stretching typically causes an increasedbulging resistance of the workpiece, for example, the outer skin part.

It is therefore at least one object to specify a stretch-forming devicefor stretch forming a workpiece, a method for stretch forming aworkpiece using a stretch-forming device, and a stretch-formed part,which allow higher pre-stretching values and/or higher pre-stretching ofa workpiece. In addition, other objects, desirable features andcharacteristics will become apparent from the summary and subsequentdetailed description, and the appended claims, taken in conjunction withthe accompanying drawings and this background.

SUMMARY

A stretch-forming device is provided for stretch forming a workpiecethat has a first bending device, implemented for bending a first endarea of the workpiece. The first bending device has a first bendingstamp and a first bending shoe. In addition, the stretch-forming devicehas a second bending device, implemented for bending a second end areaof the workpiece. The second bending device has a second bending stampand a second bending shoe. Furthermore, the stretch-forming device has afirst shaft, the first shaft being mounted so it is rotatable around afirst axis and providing a first support surface for the workpiece. Inaddition, the stretch-forming device has a second shaft, the secondshaft being mounted so it is rotatable around a second axis andproviding a second support surface for the workpiece. The first bendingstamp and the second bending stamp are components of a first part of thestretch-forming device and the first bending shoe and the second bendingshoe are components of a second part of the stretch-forming device. Thefirst part and the second part are movable relative to one another. Thesecond part has a first recess and a second recess, the first bendingshoe and the first shaft being situated on opposite sides of the firstrecess and the second bending shoe and the second shaft being situatedon opposite sides of the second recess.

The stretch-forming device advantageously allows, through the tworotatably mounted shafts in the form of the first shaft and the secondshaft, which each provide a support surface for the workpiece, a nearlyfrictionless force deflection during stretch forming of the workpiece.Friction losses during the stretch forming are thus reduced andincreased force introduction into the part geometry in the form of theworkpiece is made possible, whereby increased pre-stretching can beachieved, which lies in a range of approximately 5% to approximately 7%,for example.

The higher pre-stretching causes increased bulging resistance of theworkpiece. The achieved higher bulging resistance advantageously allowsa reduction of the plate thickness of the workpiece and thus lowermaterial costs and a reduction of the part weight. In addition toallowing a lower material use and a lower part weight for the workpiece,the stretch-forming device according to the invention has the furtheradvantage that in contrast to typical deep drawing, no sheet-metalholder or holders are required. The sheet-metal holder guide andpressurized-gas springs or lifting pins are thus dispensed with and theoverall tool height can be reduced. Overall, the mentioned advantagesadditionally cause a cost savings both in the stretch-forming device andalso in the workpiece to be formed.

The first part of the stretch-forming device can be implemented asstationary and the second part of the stretch-forming device can beimplemented as movable. In a further embodiment, both the first part andalso the second part of the stretch-forming device are movable. Thesecond part of the stretch-forming device is particularly preferablyimplemented as stationary and the first part of the stretch-formingdevice is implemented as movable.

In a further embodiment of the stretch-forming device, the first bendingstamp and the second bending stamp are situated on opposite sides of thefirst part and the first bending shoe and the second bending shoe aresituated on opposite sides, corresponding thereto, of the second part.In this embodiment, the first bending device and the second bendingdevice are therefore situated on opposite sides of the first and secondpart of the stretch-forming device.

In a further embodiment, the stretch-forming device additionally has athird bending device having a third bending stamp and a third bendingshoe and a fourth bending device having a fourth bending stamp and afourth bending shoe. The first bending device and the second bendingdevice are situated on opposite sides of the first part and the secondpart and the third bending device and the fourth bending device aresituated on opposite sides, which are complementary thereto, of thefirst part and the second part.

The aforementioned embodiments advantageously allow reliable clamping ofthe workpiece between the first bending stamp and the first bending shoeand the second bending stamp and the second bending shoe and optionallythe third bending stamp and the third bending shoe and the fourthbending stamp and the fourth bending shoe on at least two or optionallyfour sides of the workpiece.

In a further embodiment of the stretch-forming device, the height of thefirst bending stamp varies over its length and/or the height of thesecond bending stamp varies over its length. The first bending stampand/or the second bending stamp may thus be adapted optimally to theworkpiece.

In a further embodiment, the orientation of the first bending stampand/or the second bending stamp is adapted to the geometry of theworkpiece. This embodiment also allows a precise adaptation of the firstbending stamp and/or the second bending stamp to the workpiece.

In a further embodiment of the stretch-forming device, a lateraldistance between the first bending stamp and the first bending shoeand/or a lateral distance between the second bending stamp and thesecond bending shoe is adapted in such a manner that a predeterminedclamping force for the workpiece is settable. A clamping force to be setfor the workpiece, in order that it does not slip out of the clamping,can thus advantageously is provided, without further components beingrequired for this purpose.

The stretch-forming device can further have a deep-drawing device havinga forming stamp and a matrix corresponding thereto. The forming stamp ispreferably a component of the first part of the stretch-forming deviceand the matrix is a component of the second part of the stretch-formingdevice. The aforementioned embodiments thus allow, in addition to theplanar stretch forming of the workpiece, deep drawing processingthereof. Particularly preferably, the first part is an upper part of thestretch-forming device and the second part is a lower part of thestretch-forming device.

In a further embodiment of the stretch-forming device, the first bendingshoe has a first clamping device and the second bending shoe has asecond clamping device, the first clamping device being situatedadjacent to the first recess and the second clamping device beingsituated adjacent to the second recess. By providing the first clampingdevice and the second clamping device, the required clamping, i.e., apredetermined clamping force for the workpiece can be provided veryprecisely, variations in the thickness of the workpiece additionallybeing able to be compensated for using the clamping devices.

The first clamping device and/or the second clamping device preferablyhave a plate and a clamping insert. The plate can comprise apolyurethane plastic and/or a hard rubber. The polyurethane plasticparticularly advantageously has a flat modulus of elasticity. In afurther embodiment, the first clamping device and/or the second clampingdevice have compressed-gas springs.

In a further embodiment of the stretch-forming device, the second parthas a first brake shoe and a second brake shoe, the first brake shoebeing situated below the first shaft and adjacent to the first recessand the second brake shoe being situated below the second shaft andadjacent to the second recess. Uncontrolled running on of the workpieceduring the forming thereof can thus advantageously be avoided. The firstbrake shoe and the second brake shoe preferably comprise a polyurethaneplastic and/or a hard rubber and/or compressed-gas springs.

A method is also provided for the stretch forming of a workpiece using astretch-forming device according to one of the embodiments, the methodhaving the following steps: the workpiece is laid at least on the firstsupport surface, the second support surface, the first bending shoe, andthe second bending shoe. In addition, the first part is moved relativeto the second part in such a manner that the first end area of theworkpiece is clamped between the first bending stamp and the firstbending shoe and the second end area of the workpiece is clamped betweenthe second bending stamp and the second bending shoe. Furthermore, theworkpiece is pre-stretched. The method has the advantages alreadymentioned in connection with the stretch-forming device according to theembodiments. These are not listed once again at this point to avoidrepetitions. The workpiece is preferably a deep-drawn plate or analuminum plate. In one embodiment of the method, after thepre-stretching of the workpiece, the workpiece is deep drawn if thestretch-forming device has a deep-drawing device. This method isparticularly suitable for workpieces which are to have a small drawingdepth.

Furthermore, a stretch-formed part of an outer skin of a motor vehicleis provided that has pre-stretching VR in a range of approximately4%≦VR≦8%, preferably approximately 5%≦VR≦7%, and more preferablyapproximately 6%≦VR≦7%. The stretch-formed part has the advantagesalready mentioned in connection with the stretch-forming device, whichare not listed once again at this point to avoid repetitions. Thestretch-formed part is particularly preferably produced using a methodfor stretch forming a workpiece according to one of the above-mentionedembodiments. In one embodiment, the stretch-formed part is a deep-drawnplate or an aluminum plate, which preferably has a small drawing depth.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and:

FIG. 1 shows a stretch-forming device for stretch forming a workpieceaccording to an embodiment;

FIG. 2A shows a schematic perspective illustration of the first part ofthe stretch-forming device according to FIG. 1;

FIG. 2B shows a schematic perspective illustration of the second part ofthe stretch-forming device according to FIG. 1;

FIG. 3A through FIG. 3E show a method for stretch forming a workpieceaccording to an embodiment; and

FIG. 4A through FIG. 4D shows a method for stretch forming a workpieceaccording to a further embodiment.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit application and uses. Furthermore, there is nointention to be bound by any theory presented in the precedingbackground or the following detailed description.

FIG. 1 shows a stretch-forming device 1 for stretch forming a workpiece2 according to an embodiment of the invention in cross-section. Thestretch-forming device 1 has a first bending device 3, which isimplemented to bend a first end area 4 of the workpiece 2. For thispurpose, the first bending device 3 has a first bending stamp 5 and afirst bending shoe 6. In addition, the stretch-forming device 1 has asecond bending device 7, which is implemented to bend a second end area8 of the workpiece 2. For this purpose, the second bending device 7 hasa second bending stamp 9 and a second bending shoe 10.

Furthermore, the stretch-forming device 1 has a first shaft 11, thefirst shaft 11 being mounted so it is rotatable around a first axis andproviding a first support surface 12 for the workpiece 2. In addition,the stretch-forming device 1 has a second shaft 13, the second shaft 13being mounted so it is rotatable around a second axis and providing asecond support surface 14 for the workpiece 2. The respective axesaround which the first shaft 11 and the second shaft 13 are mounted sothey are rotatable are situated perpendicular to the plane of thedrawing in the embodiment shown and are not shown in greater detail inFIG. 1.

The first bending stamp 5 and the second bending stamp 9 are componentsof a first part 15 of the stretch-forming device 1 and the first bendingshoe 6 and the second bending shoe 10 are components of a second part 16of the stretch-forming device 1. The first part 15 and the second part16 are movable relative to one another. In the embodiment shown, thesecond part 16 is stationary and the first part 15 is movable.

The first part 15 is an upper part of the stretch-forming device 1 inthe embodiment shown and the second part 16 is a lower part of thestretch-forming device 1. The second part 16 has a first recess 17 and asecond recess 18. The first bending shoe 6 and the first shaft 11 aresituated on opposite sides of the first recess 17 and the second bendingshoe 10 and the second shaft 13 are situated on opposite sides of thesecond recess 18. In the embodiment shown, the first bending shoe 6forms a lateral surface of the first recess 17 and the second bendingshoe 10 forms a lateral surface of the second recess 18.

The first bending stamp 5 and the second bending stamp 9 are situated onopposite sides of the first part 15 and the first bending shoe 6 and thesecond bending shoe 10 are situated on opposite sides, correspondingthereto, of the second part 16. The first end area 4 of the workpiece 2is situated on the same side as the first bending stamp 5 and the firstbending shoe 6 and the second end area 8 of the workpiece 2, which isopposite to the first end area, is situated on the same side as thesecond bending stamp 9 and the second bending shoe 10.

Furthermore, the stretch-forming device 1 has a deep drawing device 19having a forming stamp 20 and a matrix 21 corresponding thereto. In theembodiment shown, the forming stamp 20 is a component of the first part15 of the stretch-forming device 1 and the matrix 21 is a component ofthe second part 16 of the stretch-forming device 1. The workpiece 2 is,for example, a board or a plate, in particular a deep-drawn plate or analuminum plate.

FIG. 2A shows a schematic perspective illustration of the first part 15of the stretch-forming device 1 according to FIG. 1 and FIG. 2B shows aschematic perspective illustration of the second part 16 of thestretch-forming device 1 according to FIG. 1. Components having the samefunctions as in FIG. 1 are identified by the same reference numerals andare not explained in greater detail hereafter. As may be seen in FIG. 2Aand FIG. 2B, the first bending stamp 5 and the second bending stamp 9 aswell as the first bending shoe 6 and the second bending shoe 10 eachhave a diagonal shape in their longitudinal directions in the embodimentshown. The orientation of the first bending stamp 5, the second bendingstamp 9, the first bending shoe 6, and the second bending shoe 10 isthus adapted to the geometry of the workpiece (not shown in FIGS. 2A and2B). In addition, it is also possible that the height of the firstbending stamp 5 varies over its length and/or the height of the secondbending stamp 9 varies over its length.

Furthermore, plain bearings 24, 26, 27, and 28 are shown in FIG. 2B, inwhich the first shaft 11 and the second shaft 13 are mounted so they arerotatable. The plain bearings 24 and 26 are situated on the two oppositeends of the first shaft 11 and the plain bearings 27 and 28 are situatedon the two opposite ends of the second shaft 13. Furthermore, inaddition to the respective plain bearing situated at the ends of thefirst shaft 11 and the second shaft 13, support bearings may beprovided, which are situated between the two opposite ends of therespective shaft.

FIG. 3A through FIG. 3E show a method for stretch forming a workpiece 2using the stretch-forming device 1 according to an embodiment.Components having the same functions as in the preceding figures areidentified by the same reference numerals and are not explained ingreater detail hereafter. In FIG. 3A through FIG. 3E, only a subarea ofthe stretch-forming device 1 is shown in each case, in particular thepart which contains the first bending device 3 and the first shaft 11.The second bending device (not shown) and the second shaft (also notshown) are situated as shown in FIG. 1.

In a first step of the method according to an embodiment, the workpiece2, for example, in the form of a board, is laid on the first supportsurface 12 of the first shaft 11 and the second support surface of thesecond shaft and the first bending shoe 6 and the second bending shoe.FIG. 3A shows the stretch-forming device 1 after the just-mentionedmethod step.

In a further step, the first part 15 is lowered in the direction towardthe second part 16 and the workpiece 2 is thus clamped using a part ofthe first end area 4 between the first bending stamp 5 of the first part15 in the form of an upper part and the first bending shoe 6 of thesecond part 16 in the form of a lower part. In addition, the workpiece 2is clamped using a part of the second area between the second bendingstamp and the second bending shoe. FIG. 3B shows the stretch-formingdevice 1 after the just-mentioned method step.

In a further step, the first part 15 is lowered further. As the firstbending stamp 5 is moved lower, which is connected thereto, the clampedworkpiece 2 is also drawn downward and pre-stretched via the first shaft11 and the second shaft. Since the first shaft 11 and the second shaftare mounted so they are rotatable, nearly frictionless force deflectionis possible, whereby the highest possible pre-stretching, for example,in a range from approximately 5% to approximately 7%, can be achieved.In order that the workpiece 2 does not slip out of the clamping, aclamping force ascertained depending on the material quality of theworkpiece 2, for example, depending on plate quality, is provided. Theclamping force is set via the distance between the first bending stamp 5and the first bending shoe 6 or the second bending stamp and the secondbending shoe. For this purpose, this distance is adapted in such a waythat the corresponding clamping force can be provided via a segmentedsurface pressure. FIG. 3C shows the stretch-forming device 1 after thejust-mentioned method step.

After the pre-stretching to be achieved has been reached, the staticfriction between the workpiece 2 and the first bending stamp 5 or thesecond bending stamp is overcome and the workpiece 2 can run on. Theclamping effect is thus reduced because of a decreased clamping surface,which results through the material retraction, and the forming of thepart into the form of the workpiece 2 begins. The workpiece 2 can run onfurther because of the reduced clamping force. The correspondingdeep-drawing process is shown in FIG. 3D and FIG. 3E.

FIG. 4A through FIG. 4D show a method for stretch forming a workpiece 2using a stretch-forming device 23 according to a further embodiment.Components having the same functions as in the preceding figures areidentified by the same reference numerals and are not explained ingreater detail hereafter. Only a subarea of the stretch-forming device23 is shown in each of FIG. 4A through FIG. 4D, in particular the partwhich contains the first bending device 3 and the first shaft 11. Thesecond bending device (not shown) and the second shaft (also not shown)are situated as shown in FIG. 1.

The method according to this embodiment is performed essentiallycorresponding to the method which was shown in FIG. 3A through FIG. 3E.The stretch-forming device 23 differs from the stretch-forming device 1in that the first bending shoe 6 has a first clamping device 22 and thesecond bending shoe has a second clamping device, the first clampingdevice 22 being situated adjacent to the first recess 17 and the secondclamping device being situated adjacent to the second recess. In theembodiment shown, the first clamping device 22 is implemented as aclamping insert 30, which is pre-tensioned using a plate 29 made ofpolyurethane plastic. The second clamping device is also implemented asa clamping insert pre-tensioned using a plate made of polyurethaneplastic. The clamping force which is used depending on the quality ofthe workpiece 2 is set using the pre-tensioned clamping inserts.Furthermore, possibly existing thickness tolerances of the workpiece 2may be compensated for using the clamping insert.

In addition, the second part 16 has a first brake shoe 25 and a secondbrake shoe, the first brake shoe 25 being situated below the first shaft11 and adjacent to the first recess 17 and the second brake shoe beingsituated below the second shaft and adjacent to the second recess. Inthe embodiment shown, the first brake shoe 25 is implemented as aninsert 32 which is pre-tensioned using a plate 31 made of polyurethaneplastic. The second brake shoe is also implemented as an insert which ispre-tensioned using a plate made of polyurethane plastic. On the onehand, the controlled intake during the forming is achieved using thepre-tensioned inserts; on the other hand, the material of the workpiece2 is pre-stretched up to a defined drawing depth. Furthermore, possiblyexisting thickness tolerances of the workpiece 2 may be compensated forusing the inserts.

In a first step of the method according to this embodiment, theworkpiece 2, for example, in the form of a board, is laid on the firstsupport surface 12 of the first shaft 11 and the second support surfaceof the second shaft as well as the first bending shoe 6 and the secondbending shoe. FIG. 4A shows the stretch-forming device 23 after thejust-mentioned method step.

In a further step, the first part 15 is lowered in the direction towardthe second part 16 and the workpiece 2 is thus clamped using a part ofthe first end area 4 between the first bending stamp 5 of the first part15 in the form of an upper part and the first clamping device 22 in theform of a pre-tensioned clamping insert of the second part 16 in theform of a lower part. In addition, the workpiece 2 is clamped using apart of the second end area between the second bending stamp and thesecond clamping device in the form of a pre-tensioned clamping insert.FIG. 4B shows the stretch-forming device 23 after the just-mentionedmethod step.

In a further step, the first part 15 is lowered further. As the firstbending stamp 5 moves lower in connection therewith, the clampedworkpiece 2 is also drawn downward and pre-stretched via the first shaft11 and the second shaft. Since the first shaft 11 and the second shaftare mounted so they are rotatable, nearly frictionless force deflectionis possible, whereby the highest possible pre-stretching, for example,in a range of approximately 5% to approximately 7%, can be achieved. Inorder that the workpiece to does not slip out of the clamping, aclamping force is provided, which is ascertained depending on thematerial quality of the workpiece 2, for example, depending on the platequality. The clamping force is provided via the pre-tensioned platesmade of polyurethane plastic behind the clamping insert. FIG. 4C showsthe stretch-forming device 23 after the just-mentioned method step.

After the pre-stretching to be achieved has been reached, the firstbending stamp 5 and the second bending stamp run over the respectiveclamping inserts, whereby the clamping force is reduced, and the formingof the part into the form of the workpiece 2 begins. The workpiece 2 canrun on because of the reduced clamping force. In order to preventuncontrolled running on, pre-tensioned brake shoes as already describedabove are installed below the first shaft 11 and the second shaft. Thecorresponding deep-drawing process is shown in FIG. 4D.

While at least one exemplary embodiment has been presented in theforegoing summary and detailed description, it should be appreciatedthat a vast number of variations exist. It should also be appreciatedthat the exemplary embodiment or exemplary embodiments are onlyexamples, and are not intended to limit the scope, applicability, orconfiguration in any way. Rather, the foregoing summary and detaileddescription will provide those skilled in the art with a convenient roadmap for implementing an exemplary embodiment, it being understood thatvarious changes may be made in the function and arrangement of elementsdescribed in an exemplary embodiment without departing from the scope asset forth in the appended claims and their legal equivalents.

1. A stretch-forming device for a stretch forming of a workpiece,comprising: a first bending device adapted to bend a first end area ofthe workpiece, the first bending device comprising a first bending stampand a first bending shoe; a second bending device adapted to bend asecond end area of the workpiece, the second bending device comprising asecond bending stamp and a second bending shoe; a first shaft mountedand rotatable around a first axis and adapted to provide a first supportsurface for the workpiece; a second shaft mounted and rotatable around asecond axis and adapted to provide a second support surface for theworkpiece; a first part comprising the first bending stamp and thesecond bending stamp; and a second part formed of a first bending jawand the second bending shoe, the second part movable relative to thefirst part and the second part comprising a first recess and a secondrecess, wherein the first bending shoe and the first shaft are situatedon opposite sides of the first recess and the second bending shoe andthe second shaft are situated on opposite sides of the second recess. 2.The stretch-forming device according to claim 1, wherein the firstbending stamp and the second bending stamp are situated on oppositesides of the first part, and the first bending shoe and the secondbending shoe are situated on opposite sides of the second part.
 3. Thestretch-forming device according to claim 1, wherein a height of thefirst bending stamp varies over a length of the first bending stamp. 4.The stretch-forming device according to claim 1, wherein a height of thesecond bending stamp varies over a length of the second bending stamp.5. The stretch-forming device according to claim 1, wherein anorientation of the first bending stamp is adapted to a geometry of theworkpiece.
 6. The stretch-forming device according to claim 1, whereinan orientation of the second bending stamp is adapted to a geometry ofthe workpiece.
 7. The stretch-forming device according to claim 1,wherein a lateral distance between the first bending stamp and the firstbending shoe is adapted such that a predetermined clamping force issettable for the workpiece.
 8. The stretch-forming device according toclaim 1, wherein a lateral distance between the second bending stamp andthe second bending shoe is adapted such that a predetermined clampingforce is settable for the workpiece.
 9. The stretch-forming deviceaccording to claim 1, further comprising a deep-drawing device thatcomprises a forming stamp and a matrix.
 10. The stretch-forming deviceaccording to claim 9, wherein the forming stamp is a first component ofthe first part of the stretch-forming device and the matrix is a secondcomponent of the second part of the stretch-forming device.
 11. Thestretch-forming device according to claim 1, wherein the first part isan upper part and the second part is a lower part.
 12. Thestretch-forming device according to claim 1, wherein the first bendingshoe comprises a first clamping device and the second bending shoecomprises a second clamping device, the first clamping device issituated adjacent to the first recess and the second clamping device issituated adjacent to the second recess.
 13. The stretch-forming deviceaccording to claim 12, wherein the first clamping device comprises aplate and a clamping insert.
 14. The stretch-forming device according toclaim 12, wherein the second clamping device comprises a plate and aclamping insert.
 15. The stretch-forming device according to claim 13,wherein the plate comprises a polyurethane plastic.
 16. Thestretch-forming device according to claim 14, wherein the platecomprises a rubber.
 17. The stretch-forming device according to claim13, wherein the plate comprises a compressed-gas spring.
 18. Thestretch-forming device according to claim 1, wherein the second partcomprises a first brake shoe and a second brake shoe, the first brakeshoe situated below the first shaft and adjacent to the first recess,and the second brake shoe situated below the second shaft and adjacentto the second recess.
 19. The stretch-forming device according to claim18, wherein the first brake shoe and the second brake shoe comprise apolyurethane plastic.
 20. The stretch-forming device according to claim18, wherein the first brake shoe and the second brake shoe comprise arubber.
 21. The stretch-forming device according to claim 18, whereinthe first brake shoe and the second brake shoe comprise a compressed-gassprings.
 22. A method for stretch forming a workpiece with astretch-forming device, the stretch-forming device comprising: a firstbending device adapted to bend a first end area of the workpiece, thefirst bending device comprising a first bending stamp and a firstbending shoe; a second bending device adapted to bend a second end areaof the workpiece, the second bending device comprising a second bendingstamp and a second bending shoe; a first shaft mounted and rotatablearound a first axis and adapted to provide a first support surface forthe workpiece; a second shaft mounted and rotatable around a second axisand adapted to provide a second support surface for the workpiece; afirst part comprising the first bending stamp and the second bendingstamp; and a second part formed of a first bending jaw and the secondbending shoe, the second part movable relative to the first part and thesecond part comprising a first recess and a second recess, wherein thefirst bending shoe and the first shaft are situated on opposite sides ofthe first recess and the second bending shoe and the second shaft aresituated on opposite sides of the second recess, the method comprising:placing the workpiece at least on the first support surface, the secondsupport surface, the first bending shoe and the second bending shoe;moving the first part relative to the second part such that the firstend area is clamped between the first bending stamp and the firstbending shoe and the second end area is clamped between the secondbending stamp and the second bending shoe; and pre-stretching theworkpiece.
 23. The method according to claim 22, wherein thepre-stretching the workpiece produces a stretch-formed part of an outerskin of a motor vehicle and the stretch-formed part has a pre-stretchingVR in a range approximately 4%≦VR≦8%.